In order to conserve crude oil and out of concern for the environment, the automotive industry is required to ramp up production of more fuel-efficient vehicles on a tight timeline. As an essential power conversion system for vehicle propulsion, the technology of the internal combustion (“IC”) engine is the focus of attention for attempting the improvement of vehicle efficiency. It is known that the technology of the IC engine has its advantages as compared with other energy conversion technologies used on vehicles.
The IC engine is a proven cost-effective technology with a long history of dominating the markets of power generation, indicating that the technology has stood firm with the test of time. Different from other indirect technologies that convert fuel in the following manner: heat energy→electrical energy→mechanical energy or chemical energy→electrical energy→mechanical energy, an IC engine can directly convert the heat energy of fuel→mechanical energy; thus, t avoids the reduction of fuel efficiency during the process of conversion. The IC engine possesses competitive ratios of power/weight and power/volume, and can also run without pollution if proper fuel is used. Even if fossil fuel is used up in the future, non-fossil fuel replacements cat be easily found. As a result, the transfer from the fossil fuel era into the non-fossil fuel era will probably occur smoothly with a convenient and inexpensive change of the power-generating method.
Unfortunately, the efficiency of an IC engine is still low. An IC engine may only run at around 30% of fuel efficiency. For decades, scientists have devoted great efforts to improve the efficiency of IC engines in deliberating of many different approaches. Currently, the general ways to improve the fuel efficiency of IC engine may be understood from Taylor's work. To have overall understanding of the current IC engine technology, Taylor conducted a systematic review of the IC engine technology. He suggested that there could be 6-15% improvement in internal combustion fuel efficiency in the coming decade. Taylor stated that developments beyond the next decade were likely to be dominated by four topics: emission legislation and emission control, new fuels, improved combustion; and advanced concepts for energy saving
IC engines could achieve higher efficiency to meet the requirements of the raised standards with the development of new technology as listed in the above four topic. However, upon an investigation of the prior art, it is believed that at least one more topic may also be considered as an important way to improve the performance of IC engines, that is, the topic of increasing the fuel efficiency of IC engines. It is known that the fuel efficiency of an IC engine is not only related to the efficiency of the engine itself but also related to the efficiency of the operating environment. If a high-efficient engine is arranged to run in an improper operating environment, the fuel efficiency in application will remain low.
It is acknowledged that an IC engine could run at its most efficient state at certain range of engine speeds measured in revolutions per minute (RPM) which produces peak power, and maximizing fuel efficiency by always allowing the engine to run at the optimum RPM. For decades, scientists have tried to find the best way to use the concept for better fuel efficiency. The trend of how to use the working properties of an IC engine to achieve optimal fuel efficiency can be understood by the following concise discussions of the prior art.
Osman et al, noted that it was possible to improve the efficiency of conventional vehicles by intelligent control of the drivetrain. They therefore modeled IC engines using neural networks for fuel saving by intelligent control of the transmission. The initial results they presented showed that considerable fuel savings could be achieved by intelligent selection of optimal shift points.
Maugham et al. considered that, due to low frictional and pumping losses, a gasoline engine's fuel efficiency could be maximized at low speed, high torque conditions. They indicated that a continuously variable transmission (“CVT”) allowed a drivetrain controller the freedom to develop a required output power at a range of engine torque and speed conditions. This flexibility can be used to maximize fuel efficiency. Since controlling a gasoline engine in this range could compromise transient vehicle response, the preliminary work had been taken by them to investigate the potential of charge dilution to control steady state engine torque. The outcomes of their research showed the potential to maintain economy gains with a CVT drivetrain while improving a vehicle's drivability.
Sasaki introduced Toyota's new hybrid drivetrain system that included a drivetrain that consisted of a planetary gear mechanism for dividing the drive force and two motor/generators. Such a system is based on the concept that uses computer control to optimize engine fuel consumption and to minimize exhaust emissions.
Ariyono et al. presented their research about the continuously variable transmissions. They considered that CVT could provide a wider range transmission ratio, good fuel economy, smoothly shifting of ratio, and excellent drivability. They also noted that, with CVT, it was possible to maintain a constant engine speed based on either its optimum control line or maximum engine power characteristic. With their work, the use of Adaptive Neural Network Optimization Control (“ANNOC”) was brought in to indirectly control the engine speed by adjusting pulley CVT ratio.
Hayashi et al. developed a transmission controller for an automobile to deal with the issue of variable loads. Such a development is, actually, an automated manual gear-shift system. Neuro and fuzzy methods are adopted for the controller and the interface between a vehicle operator and an automobile to make the operator feel comfortable even when automobile loads change while traveling.
Scherer et al. introduced the six-speed automatic transmission for passenger cars developed by ZF Freidrichshafen AG (“ZF”) in 2001. They mentioned that, with regard to the increasing requirements especially in reduction of CO2 emissions, a new eight-speed transmission is now under development in ZF. The main targets for this transmission family are a further significant reduction in fuel consumption and emissions, good driving performance and state-of-the-art driving comfort. The authors considered that the new developments showed that the technology of “conventional” automatic transmissions with torque converter and planetary gear sets still presented a lot of potential.
Lorenz et al. described that the BMW 750i had interconnected electronic systems to control the engine and the automatic transmission. They illustrated the concept and interplay of the drivetrain, the driving stability electronics, and the individual functions, and also described the measures taken to make the drive-management system reliable.
Bednarek et al. introduced GM's new six-speed front wheel drive (FWD) family of automatic transmissions. They reported that the new transmission had a wide overall ratio spread of 6.1:1 which allowed for optimum adaptation of the drivetrain for various categories of vehicles. The authors considered that the use of the torque converter lockup clutch with electronically controlled slip could lead to the optimum use of the fuel in vehicles.
The following prior art also conducted an investigation about existing transmission and drivetrain technologies with corresponding analysis of fuel efficiency and prediction of the trends of future development.
Höhn described the future trends of automotive drivetrains in Europe. He pointed out that automotive transmissions for passenger cars in Europe have changed continuously in the last 20 years. The ratio range for transmissions has increased more and more. He believed that the trend to more gears for automatic transmissions and manual gear boxes would be on-going. He predicted that, in the future, different concepts would be developed, and semi-automated manual gearboxes, CVTs, and hybrid systems would be applied even though in the past only manual gearboxes and automatic transmissions were on the market.
Wu and Sun introduced five major automated transmissions in international automobile markets regarding their working principles and development history as well as the state-of-the-art of the automated transmissions. Advantages and disadvantages of automated transmissions were compared, and development perspective was presented. For convenience, representative manufacturers and the application status in the Chinese market are listed in an accompanying information disclosure statement.
Wu and Sun also investigated the development history and research status of the continuously variable transmission. Particularly, they introduced the basic structure of metal v-belt CVT, and compared the structure and working principle of the metal belt and chain. With their research, the principle and performance of the mechanical-hydraulic control system and electro-hydraulic control system, especially the slip control strategy developed recently were explained, and future CVT development trend were predicted.
Buscemi discussed that, today, drivers could have more transmission options than before as automatics and manuals are accompanied by automated manuals (“AMT”), dual clutch transmissions (“DCT”), and continuously variable transmissions. The author stated that the most important objective and the main goal for transmission engineers would be to improve fuel efficiency and to perk up overall drivetrain efficiency by reducing drag losses.
Srivastava and Hague studied the significant developments of vehicle transmissions over the last two decades. They considered that a CVT would offer a continuum of gear ratios between desired limits, which would consequently enhance the fuel economy and dynamic performance of a vehicle by better matching the engine operating conditions to the variable driving scenarios. They mentioned that the potential of CVT had not been realized in the mass production of vehicle although it could play a crucial role to improve vehicle fuel economy, and discussed the challenges and critical issues for future research on modeling and control of CVTs.
Gilmore emphasized that efficiency goals could represent one of the key factors governing drivetrain choice. With specifying three developments, the conventional four-speed manual or automatic transmission was competed, and the fuel consumption associated with continuously variable ratio and infinitely variable ratio automobile transmissions were simulated in the situations of urban and highway constant-speed operation.
Amann reviewed the historical growth in average drivetrain efficiency of the US passenger-car fleet, and found that imposition of emission control caused a temporary retreat, but with the introduction of the catalytic converter, growth was restored to bring a dramatic decrease in exhaust emissions. He mentioned the dominant position of the gasoline engine and predicted the continuous improvement of transmission.
The prior art shows the essential ways to apply the aforementioned concept to improve the efficiency of an IC engine, i.e., the vehicle fuel efficiency, with the development of different control schemes and the introduction of new multiple-speed AMTs, DCTs, and various CVTs. However, through study of the prior arts, it is clear that several issues remain to be resolved.
It is noted that the prior art was developed with the concept to approach the optimal fuel efficiency by controlling the IC engine to run in the RPM range around the maximum power since the range is close to the congregation of the minimum fuel consumption. Such a concept may not always work because of the following reasons.
The torque decreases with the increase of speed after the maximum torque is reached. Since the output power is the product of the two, the maximum power may almost stay unchanged in a large range of RPMs. This means that there are many RPMs of maximum power; therefore, it is difficult to use the above-mentioned concept to approach optimal fuel consumption.
From the study of the working properties of IC engines, it has been found that there are basically two different IC engine working profiles. The first profile is a congregation of the minimum fuel consumption RPM close to the maximum output torque RPM. The second profile is no congregation of the minimum fuel consumption RPM near the maximum output torque RPM, and the fuel consumption rate simply rises with the increase of RPM. This means that the concept discussed in the prior art can only work with the first profile.
The prior art did not identify the necessary condition to ensure that the control method is workable to control a particular IC engine in practice.
Even though the prior art described that the use of CVTs could increase the fuel efficiency of an IC engine, they did not specify if CVTs could handle power transmission with satisfactory mechanical efficiency.
The prior arts did not present a case-emphasized propulsion control scheme to satisfy all primary needs for every single vehicle yielding the statistic data of the survey that how the vehicle could be used.
There is no claim of a n-ratio automatic transmission formed by n-pair of single-stage gears that has high mechanical efficiency to conveniently approach the case-emphasized propulsion control scheme with a generalized solution to take advantage of the IC engine's working characteristics yielding the data of survey for all different cases, and at the same time, to provide a comfortable operation similar to the automatic transmission to make a gear transmission an almost CVT.
It is important to note that, so far, the fuel efficiency of an IC engine equipped on a vehicle coupled with a drivetrain (either traditional gearbox/clutch transmission or CVT) is yet to be satisfactory. It is considered that if the above issues can be resolved, it could lead to more efficient fuel consumption. In such a situation, it is necessary to comprehensively understand the IC engine working properties to conduct further development.